Solvent and process for dewaxing mineral oils



Patented Mar. 14, 1939 PATENT OFFECE SOLVENT AND PROCESS FOR DEWAXINGMINERAL OILS Donald S. McKittrick, Henriques, Berkeley,

(lakland, and Hilary J. Calif., assignors to Shell Development Company,San Francisco, Oalifl, a corporation of Delaware No Drawing. ApplicationJune 20, 1936 Serial No. 86,354

4 Claims.

This invention relates to a process for separating various waxysubstances from mineral oil containing the same. More particularly, itpertains .to an improved diluent for decreasing the vis- 5 cosity of theoil and the solubility of the wax, while improving the solubility of theoil in the liquid phase produced when solid wax is separated from oil,whereby the wax and the oil are separated more readily than has beenpossible heretofore.

Mineral oils naturally contain varying amounts of Waxy hydrocarbons,often designated as paraffin wax or petrolatum and hereinaftergenerically designated as waxes, which at normal or ele- 15 vatedtemperatures are dissolved in the liquid hydrocarbons, hereinafterreferred to as oil, but which solidify or become extremely viscous atlower temperatures. If these oils are to be used as lubricants at thesereduced temperatures the solidified or highly viscous wax causes the oilto resist flow. When the wax content is too low to impede flow, it may,nevertheless, cause cloudiness in the oil. It becomes necessary,therefore, to effect the removal of all or some of the wax.

The first step in such a process is the creation of conditions underwhich the wax has a minimum solubility in the phase containing the oil.This wax probably contains aliphatic hydrocarbons and/or naphthenic andother hydrocarbons; it belongs to the group of components of petroleumwhich are the least soluble in selective solvents. The removal of waxmay be accomplished by any of several methods.

In accordance with one of these methods, the oil is chilled to atemperature substantially below that at which the wax solidifies and thewax is separated from the liquid oil by mechanical means. As animprovement in such a process, it is common to add a non-selectivediluent, such as liquefied, normally gaseous hydrocarbons, lightnaphthas, etc., to the oil to reduce its viscosity and facilitate themechanical separation.

In another method, a selective solvent medium is added to the initialoil, and the wax-free components of the initial oil are dissolvedtherein. This method necessitates the use of a solvent medium which issufiiciently selective to dissolve the oil at a suitable dewaxingtemperature without dissolving the wax. The ordinary selective solventswhich are employed for example in liquid-liquid solvent extractionprocesses to dissolve certain types of oils which are less parafiinic innature from those which are more parafiinic in nature are not in generalsuitable for use as 55' solvent media, because in dewaxing processes thesolvent is used to dissolve the desired oil which, in the case ofextraction processes, it is desired to recover in the liquid phaseinsoluble in the solvent.

Certain of such selective solvents may, however, be employed fordewaxing purposes by adding thereto a quantity of a secondary solvent,sometimes referred to as a solubility enhancing solvent. The addition ofthe secondary solvent improves the miscibility of oil and selectivesolvent, but in most cases destroys the selectivity of the resultingmixture between wax and oil, so that the resulting mixture of primaryselective solvent and solubility enhancing solvent is to a great extentequivalent to a non-selective diluent, and functions only to reduce theviscosity of the chilled oil.

We have discovered that the yield of dewaxed oil for a given pour pointand/or the pour point of the dewaxed oil can be materially improved byseparating solid wax from oil in the presence of a. dewaxing solventcontaining a mono or di-alkyl ether of ethylene glycol or diethyleneglycol as a selective solvent, and a solubility enhancing agent selectedfrom the group: aliphatic alcohols containing one oxygen atom and fromfour to eight carbon atoms, aliphatic ethers containing one oxygen atomand from four to eight carbon atoms and aliphatic ketones and aldehydescontaining from four to ten carbon atoms. Specific examples of theselective solvents which may be employed according to the presentinvention are: ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol mono propyl ether, ethylene glycoldimethyl ether, ethylene glycol diethyl ether, diethylene glycolmonoethyl ether, diethylene glycol mono propyl ether, diethylene glycolisobutyl ether. The alkyl chains may be either straight or branchedchain. We prefer, however, to employ the lower mono-alkyl others, i. e.,monoalkyl ethers of ethyene glycol having from three to six carbonatoms, and monoalkyl ethers of dietlrvlene glycol having from five tonine carbon atoms.

While any solubility enhancing solvent falling within the above groupmay be employed, it will generally be desirable to select a solubilityenhancing solvent which is best suited to the oil being dewaxed, to theoperating temperature, to the desired results, and to the particularether employed as the selective solvent. Thus, when dewaxing highlyaromatic oils, such as western oils, and/or when operating at relativelyhigh dewaxing temperatures, and/or when a low pour point is desired,and/or when using a high molecular selective solvent, it is preferableto select those of the above secondary solvents which have fewer carbonatoms, e. g., butyl alcohol, and diethyl ketone. On the other hand,whenworking with more parafiinic oils, such as Pennsylvania oils, and/orwhen operating at lower dewaxing temperatures, and/ or when a higheryield is desired, and/or when using a low molecular selective solvent,it is preferable to employ solubility enhancing agents with more carbonatoms.

We have, moreover, found that the secondary and tertiary alcohols areeminently superior to the primary alcohols, the tertiary alcohols beingslightly superior to the secondary alcohols. Specific examples of thesepreferred solubility enhancing solvents are: Secondary and tertiarybutyl amyl and hexyl alcohols, dietheyl ether, ethyl isobutyl ether,methyl ethyl ketone, methyl butyl ketone, ethyl isopropyl ketone, methyltertiary butyl ketone, valeraldehyde, and octylaldehyde.

Suitable solvent mixtures may, for example, consist of from 10 to 60% ofa selective solventlike ethylene glycol monomethyl ether, and from 40 to90% of a solubility enhancing solventlike secondary butyl alcohol.

The method which is the subject of our invention relies primarily on thepeculiar properties of the above mixtures of solvents, which were foundto create a condition in which there is a large difference between thesolubility of the solid or waxy components of the initial oil and thesolubility of the liquid or oil components thereof. In one aspect, ourmethod comprises the steps of mixing a waxy oil, such as lubricatingoil, fuel oil, etc., which may be either a residual or distillate oil,with our selective solvent and solubility enhancing solvent, andchilling the resulting mixture to a dewaxing temperature to solidify thewax, which may then be separated from the liquid portion of the systemby any mechanical means, such as filtration, cold settling, orcentrifuging, depending upon the character of the wax. Our process is,however, particularly suitable for filtration methods. The dewaxingtemperature may, for example, be slightly below the melting point of thelowest melting wax which it is desired to remove. The pour point of thedewaxed oil may in certain cases be slightly high er than the dewaxingtemperature, but by adjusting the proportions of selective solvent andsolubility enhancing solvent, as described below, pour points which areas much as 20 F. below the dewaxing temperature may be obtained.

It is desirable, although not essential, to pro- C. to 100 C., orhigher, and then cooling it slowly The chilling, when used, is not onlyfor the purpose of merely freezing out the wax, as in the processes nowpracticed, but also for the purpose of increasing the selectivity of thesolvent mixture which may have been added to the oil at a Thistemperature at which it is less selective.

feature distinguishes our process from those in which the wax is causedto separate by mere chilling, and a non-selective solvent or diluent isadded to reduce the viscosity of the oil and to expedite the separationof the solidified wax. In this connection it should be noted that theabove selective solvents become more selective at lower temperatures.

Under certain conditions the wax which is precipitated according to theprocedure outlined above, either with or without chilling duringprecipitation, may be so soft as to make its separation diflicult. It isin these cases advantageous to chill the precipitated wax during or justprior to its final removal to harden it. Subsequently the solventmixture is removed from the separated liquid by any means, such asdistillation.

To be filterable or otherwise separable from the oil-solvent phase, thewax particles must be of suitable form, size and strength and/orhardness. We have found that if in precipitating the wax a very smallamount of liquid oil is separated out from the oil with the waxcrystals, clusters of crystals are formed and the separation of wax fromthe solution of oil and solvent is facilitated. The amount of oilnecessary for this purpose depends upon the size of the initial waxcrystals, which, in turn, depends among other things, upon the nature ofthe wax and the rate of chilling. In processes in which the crystals areextremely small and the wax content of the initial oil is high, as muchas 1.0% of the main body of the oil may be desirable. In other cases, aswhen the crystals are larger, and/or when less Wax is present, lesseramounts of oil may be desirable, and it is frequently practical topermit substantially no oil to be separated out with the wax. Since theseparation of an excess quantity of oil reduces the yield of dewaxedoil, it is desirable to control the amount of the oil which is separatedwith the wax, as described below.

A convenient method of controlling this amount of oil is to regulate theconcentration of the solubility enhancing solvent in the solventmixture. The necessary dilution ratio, i.e., the ratio of the combinedsolvent mixture to the initial oil, is generally determined by theviscosity of the oil and the solvent mixture and by the selectivity ofthe solvent mixture at the dewaxing temperature, and is made as low aspossible with a view of conserving the solvent. When using the newsolvent mixtures of our invention, the dilution ratio may be much lowerthan the ratio employed with the known solvent mixtures. For example, wehave obtained excellent results when using a dilution ratio of 2:1, andeven lower ratios, such as 1:1 may be employed, although we prefer toemploy ratios of about 4:1. At very low temperatures ratios as high as8:1 may be desirable.

For a given dilution ratio, the greater the concentration of theselective solvent component of the dewaxing mixture, the more completewill be the precipitation of wax, and extremely low pour points mayoften be obtained. However, since the selective solvent has a relativelylow solvent power for the oil, the oil-solvent system will in this caseform two liquid phases, in addition to the solid wax phase, and theyield of the dewaxed oil will be extremely low. When an excess ofsolubility enhancing solvent is employed, the oil-solvent system willexist as a homogeneous liquid phase, and both pour points and yields ofdewaxed oil will be increased. Between these concentrations ofsolubility enhancing solvents, there is a composition of the combinedsolvent mixture forming a transition point between the liquidsolid andliquid-liquid-solid phase systems. The transition composition will, ofcourse, depend upon several factors, such as the dilution ratio, thedewaxing temperature, the specific solvents employed, and the characterof the oil, but may be easily determined empirically for any given situation.

We have found that when the volume concentration of the solubilityenhancing solvent in the mixed dewaxing solvent is lower than about 10%below the concentration corresponding to the transition composition, theyields of dewaxed oil are reduced quite out of proportion to theimprovement in the pour point. Moreover, we have found that the mostefiicient dewaxing operations are those which are carried out byemploying dewaxing solvent mixtures having compositions near thetransition composition, i. e., in which the concentration of thesolubility enhancing solvent fall in the range from 10% below to 20%above the transition concentration. Using greater quantities ofsolubility enhancing agent results in greater yields, but the pourpoints of the dewaxed oil will often be undesirably high.

Such solvent mixtures as are used in accord ance with the presentinvention may, for example, at temperatures of about 0 F. to 10 F., havecomplete or substantially complete solubility for oil having specificgravities between 0.90 and 0.94 and refractive indices no between 1.50and 1.52,and substantially no solvent power for wax which is solid atthe said temperatures, i. e., not more than a few tenths of one per centof Wax will be dissolved at these temperatures; such an oil may, forexample, have a viscosity of between 50 and 55 sec. Say. Univ. at 210 F.Mixtures of solvents having this characteristic may, of course, be usedalso with oils of different refractive indices, specific gravities orviscosities.

The dewaxing process may also be carried out in several steps, thecomposition of the solvent mixture being regulated, and the processbeing carried out as described in the U. S. application Serial No.45,960, filed October 21, 1935.

The yields of dewaxed oil of a given pour point and the filtration ratescan often be further increased by adding a small amount, generallybetween 0.2% and 1% of a pour point reducing substance, such astetrastearyl glucose, pentaerithrite tetrastearate, distearyl picene,cracked residues, metallic soaps, etc., to the oil, preferably, prior tochilling. Most of the pour point reducer separates from the oil with thewax, so that it is not normally effective to lower the pour point of thefiltrate, but is effective in the formation of filterable wax crystals.

The following examples illustrate the superior results obtainable withour improved solvent mixtures. For comparison, the same oil was dewaxedwith our mixtures and with acetone-benzol mixtures, the latter mixturebeing commonly regarded as standard for dewaxing mineral oils.

Example I .Several samples of a Ventura distillate having a pour pointof 95 F. were each diluted with four volumes of various solventmixtures, heated to about 100 C. for fifteen minutes, gradually cooled,and filtered at between 0 C. and 1 C. The compositions of the solventmixtures and the yields are given as per cent by volume. The results areshown in Table I.

Table I Diluent Yield of Final pour Ex .No.

p Solubility detggalxed p015; of

Selective solvent enhancing solvent Ethylene glycol Sec. butylmonomethyl ether alcohol percent percent Percent F.

Benzol Acetone percent percent Contained 10% toluene.

With dilution ratios of 1 to 4 at 0 C., the following compositions ofsolvent dewaxing mixtures corresponded to the transition point betweenone and two liquid phase systems: 35% ethylene glycol monomethyl ether,65% secondary butyl alcohol; 72% acetone, 28% benzol. It will be notedthat in each case the yield dropped off sharply when less solubilityenhancing agent was employed than corresponds to the transition pointfor the solvent in question, with a slight lowering of the pour point,but that with our dewaxing mixtures We obtained far higher yields at thetransition point than with the acetone-benzol mixture, while in eachcase the pour point was 30 F. Upon using smaller amounts of solubilityenhancing solvent, the pour point was further lowered, our solventmixtures producing higher yields than acetone-benzol mixtures. We can,therefore, obtain considerably higher yields for a given pour point,and/or lower pour points for a given yield than with the commonly usedacetonebenzol mixtures.

Example II.The distillate described in Example I was similarly dewaxedwith four volumes of various of our dewaxing diluent mixtures, thefiltration being carried out at between 20 C. and -19 C. The results areshown in Table II:

Table II Diluent Yield of Final pour Exp. No. Solubility devg'iilxedpoigrilt of Selective solvent enhancing solvent Ethylene glycol Sec.butyl monomethyl ether alcohol percent percent Percent F.

Ben2ol* Acetone percent percent 5 35 65 91 +15 6 45 55 +10- 7 50 50 90+5 8 60 40 87 O 9 70 30 65 0 10 l- 75 25 58 +5 Contained 10% toluene.

With dilution ratios of 1 to 4 at 20 C., the following compositions ofsolvent dewaxing mixtures corresponded to the transition point betweenone and two liquid phase systems: 25% ethylene glycol monomethyl ether,75% secondary butyl alcohol; acetone, 45% benzol. The superiority of ourdiluent mixtures is apparent from the above table, which shows thatdewaxed oils with considerably lower pour points can be produced withour mixtures. With the acetone-benzol mixtures, 0 F. was the lowest pourpoint obtainable. It will be noted that our new dewaxing mixtures areparticularly advantageous for low temperature dewaxing operations, i.e., those carried out below 0 C., such as 10 C., -20 C., or lower.

These examples are given only for the purpose of illustrating theefficiency of our new solvents, and not by way of limitation. Thus, theoilsolvent ratio may be varied within wide limits, and other specificselective solvents and/ or solubility enhancing solvents may beemployed. The dewaxing may be carried out either as a continuous or as abatch process. By the term effective quantity, as used in the claims, wemean a quantity of solvent mixture which will maintain sufficient oil insolution to produce its substantially complete separation from the waxwhich is precipitated, and which will produce an oil-solvent mixture ofthe desired viscosity.

We claim as our invention:

1. A selective solvent dewaxing mixture for petroleum oils, consistingof between 10 and ethylene glycol monomethyl ether and between 40 and ofa non-primary aliphatic alcohol having one oxygen atom and from four toeight carbon atoms.

' 2. A selective solvent dewaxing mixture for petroleum oils, consistingof between 10 and 60% ethylene glycol monomethyl ether and between 40and 90% secondary butyl alcohol.

3. 'The process for dewaxing mineral oils which comprises mixing the oilwith at least an equal volume of a solvent mixture containing between 10and 60% ethylene glycol monomethyl ether and between 40 and 90%secondary butyl alcohol, chilling the resulting oil solvent mixture toprecipitate wax, and removing the precipitated wax from the oil-solventmixture by mechanical means.

4. The process of dewaxing mineral oils which comprises mixing said oilwith a liquid selective solvent comprising ethylene glycol monomethylether, chilling the oil to precipitate solid wax, and mechanicallyremoving solid wax from the resulting oil-solvent mixture in thepresence of an aliphatic alcohol having one oxygen atom and from four toeight carbon atoms.

DONALD S. MCKITTRICK. HILARY J. HENRIQUES.

